Blow molding is a fabrication method for hollow thermoplastic shapes. There are two general classes of plastic products made using the blow-molding process and related machinery: packaging products and technical parts. Packaging products include such items as bottles, jars, jugs, cans, and other containers. Technical parts include automotive components such as bumpers, fuel tanks, functional fluid containers, ducting, and the like.
The blow-molding process can be of two general types: extrusion blow molding and injection blow molding. In extrusion blow molding, a thermoplastic parison is lowered from an extruder and between mold halves. The mold halves close around the parison, and the parison is then expanded against a mold cavity by introduction of a blowing gas, usually air. In injection molding, a thermoplastic material is first injection molded into a preform parison which is then transferred to a blow mold and expanded in the same manner as in an extrusion blow-molding process.
In intermittent extrusion, the molds are mounted to a common platen and the parisons are extruded by either a reciprocating screw extruder or by a ram accumulator which holds in readiness a volume of molten plastic material needed to make the next part or parts. In continuous extrusion, a molten parison is produced from an extruder die without interruption, and a segment of the parison is severed and positioned into a mold. The molds can be moved from station to station on rotating vertical wheels, on a rotating horizontal table, or with a reciprocating action. When the parison is extruded, the mold is moved under the extruder die or flow head to receive the parison segment and then is moved to a blowing station.
The positioning of the parison relative to the mold in a rotary system is relatively difficult. Therefore, many of the current blow-molding machines use the reciprocating mold concept according to which the molds are shuttled back and forth from station to station. A major drawback of the reciprocating mold concept, however, is a limitation on production rate.
A. Shuttle Machinery
Shuttle machines are either single-sided or dual-sided machines, and can be manufactured to produce one- to six-layer containers. In a single-sided machine, the mold “shuttles” under the flow head, closes to capture the parisons, then moves away from the flow head. Blow pins are then forced downward into the molds, helping to “calibrate” the necks while air is forced into the cavity to blow the container. The shuttle motion allows the bottles to be blown and cooled to the side, without interfering with the parisons, which are continually extruding from the flow head. In a double-sided shuttle machine, there is a mold on each side of the flow head, one shuttling to the right and one to the left, which generally doubles the output of a single-sided machine.
Shuttle machines may extrude single or multiple parisons, and are characterized by the number of parisons and the horizontal spacing between the parisons. For example, a “4×100” shuttle extrudes four parisons, spaced 100 mm between the centers. This would require a platen (for attaching the molds) greater in size than 400 mm, to accommodate the required mold width. The horizontal or angled shuttling distance is thus greater than 400 mm for a 4×100 shuttle machine. In general, shuttle machines up to 2×100 mm spacing are considered small machines; shuttles up to 6×100 mm spacing are considered mid-sized machines; and shuttles larger than this are typically referred to as “long-stroke” machines. Shuttle machinery is used widely in the production of personal care bottles, medical bottles, and some small industrial containers.
The steps required for a shuttle machine to blow mold a hollow plastic object can be described by the following sequence of operations. First, as the dropping parisons approach the length of the object to be blown, the mold, in an open position, shuttles sideways to a point directly under the flow head of the machine. The molds then close to capture the parison. A knife cuts the parisons directly above the molds. The knife may be either a cold knife (cutting with a sharp edge) or a hot knife (burning through the parison).
The molds shuttle away from the flow head until they are directly under the blow pin stations. If the mold movement is horizontal, the extruder head is made to bob up vertically, so that the continuously extruding parisons do not drag against the mold as it moves sideways. In some shuttle machinery, the molds shuttle down at an angle, eliminating the need for the head and extruders to bob upwards. The blow pins are forced down into the still-open necks of the containers, calibrating the necks of the containers. In most cases, the blow pins punch down onto striker plates, which form the top edge of the neck to a precise flat dimension. Air pressure is applied to blow the containers. In many cases, the blow air is turned on before the blow pins enter the open neck of the parison, to force the plastic outward and ensure a good neck formation.
After the containers have cooled, the molds open, and again shuttle under the flow head of the machine. As the molds close on the molten parisons, masking stations that are attached to the sides of the mold close over the outside of the previously blown containers, which are still held in place by the blow pins. The blow pins retract, leaving the containers held only by the masks. As the molds again shuttle sideways, the masks transfer the formed containers sideways to a punching station. Punches come forward to remove the tails, top moil, and any handle (grip) slugs away from the bottles. The bottles are then conveyed out of the machine. This may be done by transferring the bottles onto conveyor belts, by takeout devices, or by simply dropping the bottles into a chute or onto a takeaway conveyor.
Shuttle blow molding equipment offers the following advantages: (1) relative low cost compared to other extrusion blow molding machinery; (2) capable of producing a high-quality “calibrated neck” finish with blow pins; (3) in-machine trimming so that finished bottles exit the machine; (4) capable of producing bottles of all shapes, including handle ware; (5) co-extrusion capability, with up to six layers of plastic. On the other hand, shuttle machines have some limitations. Among those limitations are: (1) not cost effective for extremely high volumes; (2) reduced bottle weight consistency compared to rotary wheel machinery, due to inevitable variations among the number of unique parisons that must be extruded in shuttle equipment; (3) cycle time disadvantage when compared to reciprocating screw machines and rotary wheel machines, particularly when producing light-weight containers; and (4) complexity of the hydraulic and control systems.
B. Indexing Machinery
Horizontal rotary blow-molding machines index circumferentially spaced mold halves in steps around a vertical axis. The mold halves each capture a vertical, continuously growing parison at an extrusion station. In one machine, the flow head extruding the parison moves up away from the mold halves after the mold halves close to capture the parison. The parison is severed adjacent the top of the mold halves, the mold halves are moved away from the extrusion station, and a top blow pin is moved into the end of the captured parison at the top of the mold halves to seal the mold cavity and blow the parison. Subsequently, the flow head and dependent parison are lowered back to the initial position so that the new parison is in position to be captured by the next pair of mold halves. The blown parison cools as the mold halves are rotated around the machine, following which the mold halves open at an ejection station and the finished article, commonly a bottle, is ejected from between the mold halves. The machine includes an in-mold labeling station between the ejection station and the extrusion station for applying labels to the interior surfaces of the mold cavities.
Blowing of the captured parison is delayed until after the mold is moved away from the flow head and the blow pin has been moved into engagement with the top of the held parison. This interval of time increases the cycle time for the machine. A mechanism is required to raise and lower the flow head. Moving the flow head may move the growing parison with the ever-present risk that this movement will cause the end of the long, growing parison to shift laterally, thereby increasing the risk that the parison is not properly aligned when captured in the mold cavity.
When blow molding bottles using a blow pin entering the cavity at the top of the mold, there is a risk that the molten parison resin will gravity-flow down from the top of the cavity before the pin is extended into the cavity and confines the top of the parison against the cavity mouth. This risk is increased in a machine where the captured parison must be moved a distance away from the flow head before the blow pin is inserted down into the mouth at the top of the mold.
In another horizontal rotary blow-molding machine the parison grows down over a blow pin at the bottom of the mold halves before closing of the mold halves. The flow head is moved up above the closed mold before severing of the new parison from the captured parison. The mold is then indexed laterally to the next station without dropping and the captured parison is blown within the cavity. In a further horizontal rotary blow-molding machine, the whole turntable supporting all of the mold halves is raised and lowered during rotation as each mold captures a parison at the extrusion station.
Although horizontal rotary blow-molding machines allow for high production rates of uniform containers, there are disadvantages in the various mechanisms which, if eliminated, will result in more reliable production of high quality containers. One such problem involves the moving flow head. The parison acts as a pendulum as it dangles beneath the flow head while it is being extruded. The knives which sever the parison portion from the parison and the clamp which pinches and seals the parison cause the parison to swing when they disengage from it. Motion of the flow head tends to amplify the swinging motion of the parison, which can lead to irregularities and flaws in the containers as the mold halves close on a parison portion that is in a different position and orientation from one mold to the next.
Mold closing also affects the quality of the molded container. It is important that the molds close in precise alignment consistently and maintain the precise alignment throughout the molding process. The molds must withstand significant internal pressure without shifting or parting to ensure a quality container with the requisite uniformity of production.
Mold cooling also affects the container production. The longer the cooling time, the less likely a container will be damaged during handling upon removal from a mold. Increased cooling time must be weighed against a decrease in output, however, and it would be advantageous if longer cooling time could be realized without adversely affecting the machine output. It would also be advantageous to provide a handling mechanism for removing containers from molds which is gentle and will not damage the container when it is most vulnerable during cooling.
To overcome the shortcomings of conventional blow-molding machines, a new continuous motion neck calibrated wheel is provided. In view of the relatively large commercial demand for various types of plastic articles, it would be desirable to have a blow-molding machine that can produce high-quality articles at a relatively low cost. The present invention satisfies this desire.
An object of the present invention is to provide an improved continuous-motion blow-molding machine capable of neck calibration that is based on a wheel concept distinguished from conventional indexing or shuttle type technology. A related object is to avoid the problems encountered with conventional attempts to run neck calibrated machines continuously, capturing the parison in molds and removing containers from the molds. Another object is to overcome the relatively low output of conventional machines by producing neck calibrated containers with relatively high output.
It is still another object of the present invention to avoid start-and-stop, or indexing, of the wheel from station to station which reduces cycle time and induces stress on the components of the machine. A related object is to provide a turntable that rotates continuously, without stopping, around an endless circle. An additional object is to provide a control system that coordinates and controls operation of the various elements of the machine.
Yet another object of this invention is to retain the formed containers on the machine beyond the initial 360 degrees first tour of rotation. A related object is to use the additional time during which the containers are retained on the machine usefully, such as to add features that perform further operations on the containers. Another related object is to provide a blow pin path separate from a container path, enabling the machine to blow-mold and cool containers while further operations are completed on already-formed containers.